This invention relates in general to a transfer apparatus for use in an electrographic copier, and more particularly to a corona transfer apparatus including a transporter for repetitively circulating a receiver sheet into engagement with a moving member carrying related transferable images for transfer of such images seriatim from the member to such sheet in superimposed register to form a composite reproduction.
In making multicolor reproductions with a plain paper electrophotographic copier, for example, a multicolored original document is illuminated to form color separation images. The color separation images expose a charged photoconductive member at spaced locations along the member to form latent image charge patterns on the member corresponding to the color separation images. Exposure may take place sequentially by illuminating the document three times with white light and passing the light images respectively through primary color filters (see, for example, U.S. Pat. No. 4,072,412, issued Feb. 7, 1978 in the names of Suda et al.). Alternatively, the document may be illuminated once and the light image divided into color separation images, such as by light splitters (see, for example, U.S. Pat. No. 3,690,756 issued Sept. 12, 1972 in the name of Smith) to simultaneously expose the photoconductive member. The latent image charge patterns are developed with complementary colored marking particles (toner) to form transferable images. The transferable images are transferred from the photoconductive member to a receiver sheet in registered superimposed relation to form a multicolor reproduction.
In order to transfer the transferable images to the receiver sheet in superimposed register, the receiver sheet may be gripped and moved in a path to repeatedly bring the sheet into transfer relation with the photoconductive member at a transfer station. For example, in U.S. Pat. No. 3,999,987 (issued Dec. 28, 1976 in the name of Davis et al), the receiver sheet is gripped by a transfer roller and recirculated into engagement with the photoconductive member a number of times corresponding to the number of developed images. Transfer rollers, however, have a tendency to abrade the photoconductive member and must be continuously cleaned to prevent defects in the development of the latent image charge patterns. Further, the electrical potential of the transfer roller must be accurately adjusted to accomplish optimal transfer with different receiver materials, material thicknesses, or ambient conditions.
A transfer apparatus having a corona transfer charger overcomes the described disadvantages of transfer rollers. However, while a corona transfer charger provides a uniform transfer field, the mechanism for gripping the receiver sheet to repeatedly bring the sheet into transfer relation with the photoconductive member has been of complex construction; see, for example, the above-mentioned U.S. Pat. No. 4,072,412. The complexity results, in part, from the necessity of adequately supporting the receiver sheet without blocking the field of the charger relative to the sheet at the transfer location. Furthermore, with the known transfer apparatus using a corona transfer charger, the receiver sheet support has confined the sheet such that the sheet does not have freedom to conform to the photoconductive member during transfer. This may induce positional misregistration between subsequent transferable images transferred to such sheet so that the images are not in accurate superimposed register.